In recent years, in paper-making industry, soft and pliable printing papers, or papers having excellent flexibility, have been demanded to make it easier to turn printed matter such as books. Also soft and pliable papers comfortable to use have been demanded for sanitary papers, such as tissue papers and toilet tissues.
Patent Document 1 discloses a paper softening composition comprising lanolin or a lanolin derivative. Patent Document 2 discloses a paper softening composition comprising a urethane alcohol or a cationized product thereof as an effective component. Patent Document 3 discloses a paper softening composition comprising pyrrolidone carboxylic acid or a salt thereof. These paper softening compositions, however, do not impart sufficient flexibility to paper.
Patent Document 4 discloses a method for manufacturing softened tissue paper, wherein a di-long-chain alkyl quaternary ammonium salt is added to pulp slurry. Patent Document 5 discloses a paper softening composition comprising a di-long-chain alkyl quaternary ammonium salt, glycerol, and water or an alcohol having four or less carbon atoms. Patent Document 6 discloses a paper softening composition comprising a long-chain alkyl quaternary ammonium salt, an unsaturated fatty acid having 8 to 24 carbon atoms, and an ester of a fatty acid and pentaerythritol. However, although these softening agents impart a certain degree of flexibility to paper, they decreases the paper strength significantly.
In order to solve the problem of low paper strength, Patent Document 7 discloses a paper softening composition containing a water-soluble, heat-reactive urethane resin as an essential component. Patent Document 8 discloses a paper-quality improver containing a compound produced by reacting a polyalkyleneimine with an alkylene oxide and a higher fatty acid. The softener and the paper-quality improver, however, do not impart sufficient flexibility to paper. Patent Document 9 discloses a paper softening composition comprising an amino-ammonium salt. This softener, however, do not impart sufficient paper strength.
Therefore, no paper softening composition capable of imparting flexibility to paper without decreasing the paper strength has been obtained.    Patent Document 1: Japanese Laid-Open Patent Publication No. 53-147803    Patent Document 2: Japanese Laid-Open Patent Publication No. 60-139897    Patent Document 3: Japanese Laid-Open Patent Publication No. 7-189170    Patent Document 4: Japanese Laid-Open Patent Publication No. 63-165597    Patent Document 5: Japanese Laid-Open Patent Publication No. 4-100995    Patent Document 6: Japanese Laid-Open Patent Publication No. 7-189171    Patent Document 7: Japanese Laid-Open Patent Publication No. 6-257098    Patent Document 8: Japanese Laid-Open Patent Publication No. 2005-82949    Patent Document 9: Japanese Laid-Open Patent Publication No. 2001-355197